An elderly man who committed a string of indecent assaults against former Greens leader Bob Brown in the 1950s has been jailed for sex offences relating to a Sydney boy in the 60s.

Ian Douglas Davidson, 84, will be behind bars for at least three years after he was found guilty at Sydney District Court of eight counts of indecent assault and one of buggery committed against an 11-year-old while ­Davidson was a teacher at Sydney Grammar School.

Davidson was charged and convicted of six counts of ­indecent assault against Mr Brown, then a schoolboy, in Armidale, NSW, in 1957, court documents state.

Ian Douglas Davidson, 84, convicted of six counts of ­indecent assault against former Greens leader Bob Brown (pictured) has been jailed for sex offences relating to a Sydney boy in the 60s

Then a young teacher, Davidson failed to declare his convictions to Sydney Grammar School when it employed him, the documents say.

They also state that during the period of abuse, the victim was scared he would be in trouble if he refused Davidson's ­advances.

'It was clear from (the victim's) evidence that he was acutely conscious of other boys becoming aware of his abuse,' judge Deborah Sweeney said.